The present invention is concerned with variable capacity compressors, vacuum or other pumps or machines, and particularly those reciprocating piston compressors used in refrigeration, air conditioning systems or heat pumps or the like, including machines such as scotch yoke compressors of U.S. Pat. No. 4,838,769, wherein it is desirable to vary the compressor output, i.e., compressor capacity modulation, in accordance with cooling load requirements. Such modulation allows large gains in efficiency while normally providing reduced sound, improved reliability, and improved creature comforts including one or more of reduced air noise, better de-humidification, warmer air in heat pump mode, or the like.
The efficiency gains resulting from a compressor with capacity modulation are beneficial in a variety of commercial applications. For example, most residential refrigerators currently utilize a single capacity compressor and cycle the compressor on and off to maintain a certain temperature within the cabinet of the refrigerator. During normal operation, the temperature of the refrigerator increases due to the warmer ambient air surrounding the refrigerator or when the refrigerator door is opened or a load of perishables having a temperature greater than that of the cabinet is introduced to the refrigerator. If the temperature exceeds a preset limit, the compressor is activated to cool the cabinet of the refrigerator. To account for the higher load conditions when the door is opened or perishables are introduced to the cabinet, the cooling capacity of the compressor is necessarily greater than the minimum required to maintain a particular temperature in the ambient conditions. With this design, the compressor undergoes multiple starts and stops to respond to varying load conditions. The high number of starts and stops will shorten the life of the compressor. Additionally, operating the compressor at full capacity during periods of minimal load is inefficient.
One approach to achieving modulation of a compressor has been to switch the stroke length, i.e., stroke, of one or more of the reciprocating pistons whereby the volumetric capacity of the cylinder is changed. In these compressors the reciprocating motion of the piston is effected by the orbiting of a crankpin, i.e., crankshaft eccentric, which is attached to the piston by a connecting rod means which has a bearing in which the eccentric is rotatably mounted.
A proposed mechanism in the published art for switching stroke is the use of a cam bushing mounted on the crankshaft eccentric, which bushing when rotated on the eccentric will shift the orbit axis of the connecting rod bearing radially and parallelly with respect to the crankshaft rotational axis and thus reduce or enlarge the rod bearing orbit radius. This, in turn, changes the piston stroke accordingly. In such cam action mechanism the piston at the reduced stroke does not attain full or primary stroke top-dead-center (TDC) positioning within the cylinder. This design diminishes compression and permits considerable reexpansion of the only partially compressed refrigerant. The efficiency of the compressor is thus markedly compromised.
Certain prior art cam mechanisms are shown and described in U.S. Pat. Nos. 4,479,419; 4,236,874; 4,494,447; 4,245,966; and 4,248,053, the disclosures of which with respect to general compressor construction and also with respect to particular structures of cylinder, piston, crankshaft, crankpin and throw shifting mechanisms are hereby incorporated herein by reference in their entirety. With respect to these patents the crankpin journal is comprised of an inner and one or more outer eccentrically configured journals, the inner journal being the outer face of the crankpin or eccentric, and the outer journal(s) being termed "eccentric cams or rings" in these patents. The outer journals are rotatably mounted or stacked on the inner journal. The bearing of the connecting rod is rotatably mounted on the outer face of the outermost journal. In these patents, all journal and bearing surfaces of the coupling structure or power transmission train of the shiftable throw piston, from the crankshaft to the connecting rod are conventionally circular.
Referring particularly to the U.S. Pat. No. 4,245,966 patent, a TDC position of the piston is said to be achieved thru the use of two eccentric rings which are provided with stops to orient the cams, in the hope of achieving the TDC position. This structure is very complex, expensive, and difficult to manufacture and to assemble, in a commercial sense. Further, as stated in this patent at col. 4 lines 32-38, the operability of these two eccentrics to attain TDC is essentially by chance and is just as likely to result in a piston-valve plate crash.